In various circumstances, wells that have been drilled into the earth need to be sealed off to prevent escape of well fluids upward through the well and well annulus to the earth's surface into the sea or into another geological layer. This can be particularly important in a “sidetrack” drilling operation where a drill string is run into a pre-existing cased wellbore and is used to drill a new sidetrack wellbore through the casing wall of a pre-existing wellbore to access a new region of the subsurface. In such an operation, the well track of the pre-existing well needs to be sealed off and abandoned below the point of entry of the new sidetrack well.
In the oil and gas industry, certain standards must be met before a well can be abandoned. International ISO, EN, API and DnV standards form the guiding standards for such activities. More specific regulations and policies have also been put in place that guide sidetracking, abandonment and drilling operations. Such guidelines and policies typically include the following requirements for sealing off a well:                a. Multiple barrier seals are required, such that if a single barrier fails a second barrier exists to prevent leakage;        b. Each barrier element should be verifiable through some form of testing;        c. Permanent well barriers must be in place prior to well sidetracks, suspension and abandonment; and        d. A permanent well annular barrier should be impermeable, non-shrinking and ductile (to withstand mechanical loads/impact). It should also have long term integrity, resistance to different chemicals/substances (e.g., H2S, CO2 and hydrocarbons) and display wetting to ensure bonding to steel.        
Before commencing a drilling or well intervention operation it is necessary to document existing barriers and to determine any need for testing existing barriers or creation of additional barriers in order to comply with the industry guidelines, standards and policies. Candidate wells for such operations often lack the necessary certification and/or the required annular barriers.
Typical oil and gas wells are constructed with a casing or other lining tubing. Casing is originally installed by running a casing string, which includes the casing section to be installed, into the wellbore. The casing string is fitted with a casing shoe at its leading end to penetrate the wellbore. When the string is located at a desired installation location in the wellbore, the casing section is usually cemented in place. Cement is pumped into the inside of the casing string and down to the casing shoe. The cement is then pumped back upward toward the surface via the casing shoe into the annular space (or casing annulus) defined between the wellbore wall and an outer surface of the casing section. The cement is then left to harden, thereby fixing the casing in place. The cementation may be incomplete along the length of the casing, such that cement may only be present in the annulus in certain intervals.
When the cement in the annulus does not provide suitable or sufficient annular seals various known techniques are used to ensure that such wells are suitably sealed in line with industry regulations. These techniques are remedial in nature involving formation of new annular seals in the well. Typically, remedial operations require cutting or perforation of the casing and pumping or squeezing extra cement into the area which requires additional sealing. Such operations can be time consuming and expensive, and may damage the casing. In addition, success rates for such operations are typically not high.